"OK honey look I know that you don't understand where I'm coming from but put yourself in my shoes. If you had just driven for 10 hours in bad weather to come see me would you want to be assaulted with a laundry list of minor things that annoyed me while you were gone?"

knight_on_the_rail:indarwinsshadow: You mean that space time isn't some two dimensional plane, and that the resulting gravity isn't a product of mass acting on the surface of that plane.

Who would'a thunk.

...I hate modern explainations of science. It's as bad the "it's turtles all the way down man".

What time period do you prefer for explanations?

I like explanations that don't use analogies that aren't logical. Doesn't matter when the explanation is formed, long as it doesn't try and project an idea relative to local ideas of cultural reference. Never works.

Honestly, I think that some ideas shouldn't try and be explained...Yet. There isn't a good understanding of gravity that can be distilled down to a few lines or a stupid picture. It doesn't work.

Isn't the "rubber sheet" the quantum foam of particles that are constantly popping in and out of our frame of reference?

I would still like to know how gravity works, if changes in gravity are instantly felt everywhere or if they are propagated. For instance combining matter and antimatter results in a big badda boom and a reduction of stuff in the universe. How long until Alpha Centauri reacts gravitationally to that happening on earth?

Anyway I just wonder if Big Bertha in Seattle could bore deep enough to pierce space time. Is there another rubber sheet below this one?

BolloxReader:Isn't the "rubber sheet" the quantum foam of particles that are constantly popping in and out of our frame of reference?

I would still like to know how gravity works, if changes in gravity are instantly felt everywhere or if they are propagated. For instance combining matter and antimatter results in a big badda boom and a reduction of stuff in the universe. How long until Alpha Centauri reacts gravitationally to that happening on earth?

Anyway I just wonder if Big Bertha in Seattle could bore deep enough to pierce space time. Is there another rubber sheet below this one?

There are numerous systems set up to detect "gravity waves", though none have yet been seen. Part of this depends on whether you accept the concept of the graviton.

An interesting notion is that, of the four elementary forces - weak, small, EM, and gravity - the first three have been "shown" to be extrapolations of the same underlying "thing". Gravity, however, is still a big unknown (one of string theory's big ideas is that, if extended to 9 dimensions, gravity can be "collapsed" into the electro-weak-strong force, thus unifying all of them; this is one version of the "unified field" holy grail). Given the problems we're having with dark matter (which now seems to affect us in local spacetime, not just at interstellar distances), it's more and more likely that our entire model and concept of gravity is wrong.

As for the "rubber sheet" being the quantum foam - no. Quite the opposite, actually: the problem with merging relativity and quantum theory is that relativity demands a smooth spacetime fabric and QT forbids it (which is why you get all sorts of fun results if you try to merge the two). You have to come up with some method for "smoothing out the bumps", as it were - something that, again, string theory is trying to do.

BolloxReader:I would still like to know how gravity works, if changes in gravity are instantly felt everywhere or if they are propagated.

They propagate at the speed of light (in a vacuum, anyway, and since nearly all of the universe is vacuum...). So if the sun blinked out of existence right now, roughly eight minutes later you would see it disappear from the sky at the same instant that the Earth stopped experiencing its gravitational pull and careened off into space.

(Incidentally, if this weren't the case, highly advanced civilizations would be able to send Faster Than Light messages by annihilating large chunks of matter and antimatter, which would be enormous fun.)

obenchainr: There are numerous systems set up to detect "gravity waves", though none have yet been seen. Part of this depends on whether you accept the concept of the graviton.

Not really. Gravitational waves in general relativity don't require gravitons. Quantum gravity does, but this is irrelevant. And gravitational waves have already been detected indirectly, for which the 1993 Nobel Prize was awarded.

An interesting notion is that, of the four elementary forces - weak, small, EM, and gravity - the first three have been "shown" to be extrapolations of the same underlying "thing".

We don't know that. It's possible to write down grand unified theories that do this, but other theories do not (such as the Standard Model), and we have no evidence to distinguish them in this regard.

Given the problems we're having with dark matter (which now seems to affect us in local spacetime, not just at interstellar distances),

I have no idea what this means. Nobody ever said that dark matter "turns off" at some distance scale.

it's more and more likely that our entire model and concept of gravity is wrong.

Most astrophysicists working in the field of dark matter would still disagree with that claim.

Quite the opposite, actually: the problem with merging relativity and quantum theory is that relativity demands a smooth spacetime fabric and QT forbids it (which is why you get all sorts of fun results if you try to merge the two). You have to come up with some method for "smoothing out the bumps", as it were - something that, again, string theory is trying to do.

Quantum theory doesn't necessarily forbid a smooth spacetime, as far as we know. Some quantum theories try to quantize spacetime itself; others do not. Perturbative string theory itself is formulated in a smooth background spacetime. Non-perturbatively, we still don't know what goes on.

Ambitwistor:[obenchainr: An interesting notion is that, of the four elementary forces - weak, small, EM, and gravity - the first three have been "shown" to be extrapolations of the same underlying "thing".]

We don't know that. It's possible to write down grand unified theories that do this, but other theories do not (such as the Standard Model), and we have no evidence to distinguish them in this regard.

You might even be being a little too generous here. Although it's possible to get EM and the weak force to converge at high energies, getting the strong force to meet them is quite hard. IIRC (which I rarely do these days) that was one of the original motivations for super-symmetry theories (which, as you well know of course, are a subset of the GUTs that you mentioned). Unfortunately, the null results at the LHC are making life very tough for super-symmetry these days.

Quantum theory doesn't necessarily forbid a smooth spacetime, as far as we know. Some quantum theories try to quantize spacetime itself; others do not. Perturbative string theory itself is formulated in a smooth background spacetime. Non-perturbatively, we still don't know what goes on.

Again you're being kinder than I would be. To be honest, I don't know what string theorists are trying to do anymore since they seem to have given up on pretty much all substantive goals of the original program.

The operative word in this or any analogy is LIKE. Gravity is LIKE a sheet of rubber with a ball in the middle and other balls that will orbit it. This is true in the important point that even the lightest ball distorts the sheet somewhat making the objects mutually attract (when we, also mentally, assume no friction on the sheet)

LIKE. It shares some properties and it is easier to think of a thing you can touch than... well... Gravity.

My dislike of Sagan stems from using this poor analogy to explain GR. The book Cosmos came out in 1980 and I read it in 1981 at 11 years old and I got pissed off. Really pissed off. The Drake Equation (but no mention of the Fermi Paradox.) Hippie Dippy science without any self-criticism. My negative reaction to that book led me to get a BS in Physics and a MA in Mathematics. So I get I should thank Sagan, even though I hate his ridiculous book.

kahnzo:My dislike of Sagan stems from using this poor analogy to explain GR. The book Cosmos came out in 1980 and I read it in 1981 at 11 years old and I got pissed off. Really pissed off. The Drake Equation (but no mention of the Fermi Paradox.) Hippie Dippy science without any self-criticism. My negative reaction to that book led me to get a BS in Physics and a MA in Mathematics. So I get I should thank Sagan, even though I hate his ridiculous book.

I often think that the hardest thing for anybody to "get" about GR is that time is curved too, not just space (and I don't exclude myself from this). Even for people who grok the mathematics, the idea that time could be curved seems to be an order more counterintuitive than curved space. (If I had a dollar for every time I've read a bad explanation for why light can't escape a black hole, I'd have $17.50).

kahnzo:The problem with the analogy is that it uses gravity to explain gravity.

The problem with the analogy is that the sort of people who are attracted to physics as a career path are mostly borderline autistic personalities who refuse to accept anything which isn't perfectly literal.

/My love is not a summer's day; she's just like a summer's day//It's the sort of thing that makes sense to those of us who have figurative minds.

I have a question about Einstein's elevator thought experiment that was part of what led to General Relativity: Why is gravity special?

The thought experiment is to have two "elevators" you inside. One "elevator" is in space with no gravity, accelerating upward at 1 g. The other is at rest on Earth, like a stopped elevator; this elevator is getting pulled downward at 1 g and the ground/building/cable is pushing upward (the normal force) against the elevator at 1 g. Einstein said that you can't distinguish by experiment whether you're in the space elevator or a normal elevator, which feels intuitively true. Dropping an object while within either elevator will cause that object to fall to the floor.

My question is why is gravity special? General Relativity seems to treat gravity specially, but since I'm not a physicist, I don't understand why. Let's say you were in an elevator and you and the elevator had a positive charge. Replacing the case of the elevator on Earth, the second elevator, rather than being gravitationally attracted to a large mass, is instead attracted in space to a small mass with a strong negative charge. If the force of the electromagnetic attraction were equal to 1 g, wouldn't you also have a situation that's experimentally indistinguishable from being on Earth? You'd be pulled down at 1 g, and you'd still have the normal force pushing you upward.

Myria:My question is why is gravity special? General Relativity seems to treat gravity specially, but since I'm not a physicist, I don't understand why. Let's say you were in an elevator and you and the elevator had a positive charge.

The gravitational equivalence principle works because all objects accelerate equally under gravity, regardless of mass. In electromagnetism, the acceleration depends on the charge-to-mass ratio, and so bodies are distinguishable in that manner.

You're probably smarter than 95% of physics majors anyways, for asking a question in the first place, and for thinking about it, but there is a fundamental difference. Inertial mass and gravitational mass are the same. In other words, the m in F=ma is the same m as the m in F = mg. If you double the mass of the object in the elevator, it will experience twice as much force if the force is gravitational, but the force would be unchanged if it had been an electric force.

Myria:I have a question about Einstein's elevator thought experiment that was part of what led to General Relativity: Why is gravity special?

The thought experiment is to have two "elevators" you inside. One "elevator" is in space with no gravity, accelerating upward at 1 g. The other is at rest on Earth, like a stopped elevator; this elevator is getting pulled downward at 1 g and the ground/building/cable is pushing upward (the normal force) against the elevator at 1 g. Einstein said that you can't distinguish by experiment whether you're in the space elevator or a normal elevator, which feels intuitively true. Dropping an object while within either elevator will cause that object to fall to the floor.

My question is why is gravity special? General Relativity seems to treat gravity specially, but since I'm not a physicist, I don't understand why. Let's say you were in an elevator and you and the elevator had a positive charge. Replacing the case of the elevator on Earth, the second elevator, rather than being gravitationally attracted to a large mass, is instead attracted in space to a small mass with a strong negative charge. If the force of the electromagnetic attraction were equal to 1 g, wouldn't you also have a situation that's experimentally indistinguishable from being on Earth? You'd be pulled down at 1 g, and you'd still have the normal force pushing you upward.

Because gravity is an illusion caused by the rapid expansion of the planet. We actually are accelerating at 1G. This is why objects have the illusion of falling to the ground when in fact the ground rushes up to meet them and also explains why heavy objects fall at the same rate as lighter ones unless the air catches them.

dready zim:Because gravity is an illusion caused by the rapid expansion of the planet. We actually are accelerating at 1G. This is why objects have the illusion of falling to the ground when in fact the ground rushes up to meet them and also explains why heavy objects fall at the same rate as lighter ones unless the air catches them.

So gravity is a repulsive force, like a gas pushing itself in all directions? Why would things have to orbit to stay at a certain altitude when they could just float there at exactly the right distance not to get caught up?

dready zim:Myria: I have a question about Einstein's elevator thought experiment that was part of what led to General Relativity: Why is gravity special?

The thought experiment is to have two "elevators" you inside. One "elevator" is in space with no gravity, accelerating upward at 1 g. The other is at rest on Earth, like a stopped elevator; this elevator is getting pulled downward at 1 g and the ground/building/cable is pushing upward (the normal force) against the elevator at 1 g. Einstein said that you can't distinguish by experiment whether you're in the space elevator or a normal elevator, which feels intuitively true. Dropping an object while within either elevator will cause that object to fall to the floor.

My question is why is gravity special? General Relativity seems to treat gravity specially, but since I'm not a physicist, I don't understand why. Let's say you were in an elevator and you and the elevator had a positive charge. Replacing the case of the elevator on Earth, the second elevator, rather than being gravitationally attracted to a large mass, is instead attracted in space to a small mass with a strong negative charge. If the force of the electromagnetic attraction were equal to 1 g, wouldn't you also have a situation that's experimentally indistinguishable from being on Earth? You'd be pulled down at 1 g, and you'd still have the normal force pushing you upward.

Because gravity is an illusion caused by the rapid expansion of the planet. We actually are accelerating at 1G. This is why objects have the illusion of falling to the ground when in fact the ground rushes up to meet them and also explains why heavy objects fall at the same rate as lighter ones unless the air catches them.

That doesn't make a bit of sense. It would have the object fell in a linear path. But. If you throw a ball in the air, the planet doesn't rush up to meet it. The ball follows a mathematical curve and heads back to earth.

dready zim:Because gravity is an illusion caused by the rapid expansion of the planet. We actually are accelerating at 1G. This is why objects have the illusion of falling to the ground when in fact the ground rushes up to meet them and also explains why heavy objects fall at the same rate as lighter ones unless the air catches them.

This is genius and perhaps more true that you realize. (Senior project was on GR and involved calculating Christoffel Symbols the "old school" way.) Treating time as an "extra" dimension allows you to kind of "fold" the rapidly expanding earth back on itself.

kahnzo:Treating time as an "extra" dimension allows you to kind of "fold" the rapidly expanding earth back on itself.

One of the things that amuses me most about relativity is that mathematically, time behaves like an imaginary space dimension. And if you could actually exceed c, your space dimension in the direction of travel would become timelike and your time dimension would become spacelike (to an inertial observer). I sometimes fantasize about writing a sci-fi book in which that thought is taken seriously as a physical reality.

czetie:BolloxReader: I would still like to know how gravity works, if changes in gravity are instantly felt everywhere or if they are propagated.

They propagate at the speed of light (in a vacuum, anyway, and since nearly all of the universe is vacuum...). So if the sun blinked out of existence right now, roughly eight minutes later you would see it disappear from the sky at the same instant that the Earth stopped experiencing its gravitational pull and careened off into space.

(Incidentally, if this weren't the case, highly advanced civilizations would be able to send Faster Than Light messages by annihilating large chunks of matter and antimatter, which would be enormous fun.)

Doubtful. Annihilation releases energy, and energy has mass just like matter.